In order to manufacture a material for producing color photographic images, at least three layers are coated onto an inert support, each layer containing a light-sensitive silver halide emulsion and a photographic coupler capable of forming yellow, magenta, or cyan dye when reacted with oxidized color developing agent. The photographic coupler and the light-sensitive silver halide emulsion are suspended in gelatin, resulting in a mixture that is uniform, but solid and un-coatable at room temperature. Prior to coating, this mixture must be melted at 45.degree. C. so that it flows smoothly through the manufacturing delivery lines and onto the inert support.
In the course of a large-scale manufacturing in, the gelatinous emulsion-coupler mixture is often held at 45.degree. C. for up to 16 hours. Unfortunately, this prolonged heating, which is necessary for smooth flow of the mixture, can also cause inferior performance in the resulting photographic product, such as increased fog, reduced contrast, and speed loss. To overcome this deficiency, it is common practice to separately hold the melted light-sensitive silver halide emulsion from the melted photographic couplers and mix them only immediately prior to coating.
Although separating the melted silver halide emulsion from the melted photographic couplers effectively stabilizes the mixture and prevents increased fog, reduced contrast, and speed loss in the resulting product, it is preferable that both the silver halide emulsion and the couplers reside in a single reservoir for case of manufacturing. In the manufacture of a film product, having fewer mixtures simplifies handling and requires fewer delivery lines. Unused delivery lines can in turn be dedicated to additional layers for the purpose of augmenting photographic performance. It is therefore desirable that the light-sensitive silver halide emulsion and the photographic couplers for a given color layer be thermally stable as a single mixture.
A melted coupler-emulsion mixture for any color layer can potentially exhibit thermal instability when the silver halide emulsion and the couplers are combined and heated prior to coating. However, our experience indicates that mixtures containing the silver halide emulsion with azopyrazolone magenta masking couplers are especially prone to this problem. The use of azopyrazolone masking couplers is well known in the art. See, for example, U.S. Pat. Nos. 2,428,034; 2,434,272; 2,455,170; 2,688,539; 2,704,711; 2,808329; 3,476,560; 3,796,574; 4,427,763; 4,777,123, and EP 213,490. Masking couplers have proven useful for correcting the unwanted blue absorption typical of most magenta photographic dyes. The magenta masking coupler, which is yellow in color, has a blue absorption in non-exposed areas of the film. In exposed areas, this blue absorption is destroyed as the masking coupler reacts with oxidized color developing agent and develops into a magenta dye. The loss of blue density from the masking dye is concurrently balanced by the increase in blue density from the magenta imaging dye. Thus, the combination of the magenta imaging dye and the unreacted masking dye uniformly absorbs a low level of blue light across the imaging scale, effectively canceling the unwanted blue absorption inherent in the magenta imaging dye alone.
While yellow-colored azopyrazolone masking couplers have been used as a means of correcting the unwanted blue absorption of magenta photographic dyes, it is especially difficult to manufacture an excellent product from a single melted mixture containing both the light-sensitive silver halide emulsion and the azopyrazolone masking coupler. The thermal instability of this melted mixture is inherent in the resulting photographic product, which compared to a product manufactured from separate preparations of the emulsion and the coupler, exhibits increased fog, low contrast, and speed loss, but most particularly increased fog. Several additives have previously been proposed to mitigate fog caused by azopyrazolone masking couplers, including a low impact development inhibitor releasing coupler (U.S. Pat. No. 5,641,613) and ballasted nitroaromatic compounds (U.S. Pat. No. 5,466,568). However, these inventions primarily reduce fog generated during the development step and exhibit little to no efficacy toward stabilizing the melted emulsion-coupler mixture prior to coating.
It is desirable to manufacture a photographic element whereby a gelatinous mixture containing an azopyrazolone masking coupler can be simultaneously heated with a silver halide emulsion without incurring inferior performance in the resulting photographic product. We have discovered that certain quinones stabilize against heat-induced degradation in such a way that a gelatinous mixture containing an azopyrazolone masking coupler and a silver halide emulsion can be heated and held in a single reservoir for 16 hours without significantly affecting the performance of the resulting product, and in addition, help control undesirable increase in fog of the film during long term storage prior to exposure and development.
It is known in the art to employ certain quinones and hydroquinones for the purpose of improving photographic performance. See, for example, U.S. Pat. Nos. 4,945,031, 5,082,764, 5,104,774, and 5,264,332, which employ a diffusion-resistant quinone for decreasing the processing time for cyan couplers. These inventions, however, do not suggest the advantageous effect of combining a quinone with an azopyrazolone magenta masking coupler, and in fact, disclose that either a quinone or a hydroquinone or a combination of both is effective for the purpose of the patent. It is also known in the art to employ quinones and hydroquinones for the purpose of reducing the formation of photographic fog. For example, U.S. Pat. No. 4,277,558 relies on diffusion-resistant quinones and hydroquinones for reducing fog in combination with certain yellow dye-forming couplers. The present inventors have ascertained that disubstituted quinones taught in the art do not effectively inhibit fog formed by azopyrazolone masking couplers, although they are effective in inhibiting fog from certain yellow dye-forming couplers. Practice of any of the above-mentioned inventions, therefore, does not satisfy the objectives of the present invention.
It is a problem to be solved to provide a photographic element containing an azopyrazolone masking coupler that can be heated in the presence of a silver halide emulsion without incurring significant degradation in performance of the resulting photographic product.